Recently electronic devices have been rapidly improved to have better properties, greater functionality, and smaller sizes. To cope with the improvement of the electronic devices, electronic components to be mounted on the electronic devices have been facing increasing demands to be smaller in size and lighter in weight. Accordingly, semiconductor element packaging techniques and circuit boards to which semiconductors are to be mounted are also required to be able to give greater density, functionality, and properties to the electronic devices.
Flexible printed circuit boards (hereinafter, may be referred to as FPCs) generally have such a structure that includes (i) a metal-clad laminate on which a circuit pattern is formed, and (ii) a cover layer on the circuit pattern. The metal-clad laminate includes a thin and flexible insulating film as a substrate (base film), and a metal foil bonded to the substrate via an adhesive of various kinds by thermal compression bonding. In such three-layered flexible printed circuit boards (3-layered FPC) having three layers (insulating film, adhesive layer, and metal foil), a polyimide films and the like are used as the insulating film conventionally. This is because polyimide is excellent in heat resistance, electric properties, etc. As to the adhesive layer, thermally-curing adhesive agents of epoxy resin type, acrylic resin type, etc. are generally used.
In order to attain such an FPC greater in density, functionality, and properties, the insulating adhesive agent and the insulating film used as the components of the FPC should also be greater in properties. More specifically, the adhesive layer etc. are required to be high in heat resistance and mechanical strength, and further to be excellent in processability, adhesion, electric properties, and size stability and be low in moisture absorption.
Regarding these points, the thermally-curing resins such as epoxy resin, acrylic resin used as the adhesive layer have such conventional advantages that they are excellent in low temperature processability which allows bonding at relatively low temperatures. Further, the conventional thermally-curing resins are highly economical conventionally. However, the thermally-curing resins are insufficient conventionally in terms of the other properties such as heat resistance etc. for example.
In order to solve the problem, a two-layer FPCs have been proposed in which an adhesive layer is also made of a polyimide material (see, Patent Literature 1, for example). Note that the FPCs whose adhesive layer is made of a polyimide material may be regarded as a three-layered FPC in a strict sense, but two polyimide layers are regarded as a single layer, thereby considering the FPCs of this type as being “two-layered”. The two-layered FPC, which is greater in heat resistance, electric properties, and size stability than the three-layered FPCs whose adhesive layer is made of epoxy resin or acryl resin, are expected as components that can satisfy future demands in properties.
On the other hand, the use of a polyimide material is disadvantageously associated with high water absorption coefficient due to properties of polyimide. The two-layered FPCs are also associated with this problem. A FPC with a high water absorption coefficient would adversely affect component mounting process in which soldering is used. More specifically, moisture taken into the polyimide material from atmosphere is rapidly discharged from the polyimide material by heat applied during the component mounting process. This results in swelling or whitening of the FPC, and further leading to adhesion or electric property between members of the FPC. In order to avoid such problems associated with the post-moisture absorption solderability, it is possible that the FPC is preliminary-dried before a mounting process so as to remove moisture from the FPC; however, this measure involves a problem of productivity, since the measure causes an increase in the number of processes.
In order to solve the problem, an adhesive film it has been proposed, in which a property of thermoplastic polyimide used in the adhesive layer is controlled. Specifically, moisture content incorporated in the adhesive film is reduced by increasing a glass transition temperature of the thermoplastic polyimide contained in the adhesive film provided on one or both sides of a heat-resistant base film so as to improve the heat resistance of adhesive layer, thereby lowering the water absorption coefficient (see, Patent Literature 2 or Patent Literature 3, for example). A measure to be taken during production processes has been proposed. This is to remove moisture by preliminary-drying the adhesive film when the adhesive film and the metal foil are bonded with each other (see, Patent Literature 4, for example).
These methods improve post-moisture absorption solderability, which has been an disadvantage associated with a polyimide material. However, a lead-free soldering is becoming more likely to be employed for semiconductor packaging, because of recent increasing awareness of environmental issues. Since the lead-free soldering has a melting point approximately 40° C. higher than that of an existing eutectic solder, materials to be used in the mounting process are to be inevitably subjected to a higher temperature. Accordingly, the materials are now required higher post-moisture absorption solderability as compared to the conventional case. Further, when the material is used for the purpose of multilayer FPC, moisture is more likely to be contained inside the material due to a multi-layered structure. This results in a state where the multilayer FPC is prone to defects at a lower solder temperature than that of the monolayer FPC; that is, the material used in the multilayer FPC is required further higher post-moisture absorption solderability.